Fetal and neonatal alloimmune thrombocytopenia (FNAIT)

What is FNAIT?

Fetal and neonatal alloimmune thrombocytopenia (FNAIT) occurs when the mother’s immune system attacks her fetus’ or newborn’s platelets. Platelets are a type of blood cell that have proteins on their surface and are involved in blood clotting. If a fetus or newborn has a low platelet count then they will have an increased risk of bleeding. In most cases the effects of FNAIT are mild, however, if bleeding occurs in the brain, there may be long-term effects.

Other names for FNAIT

FNAIT is also called NAIT, AIT, FMAIT (fetal maternal alloimmune thrombocytopenia), NAT or NATP (neonatal alloimmune thrombocytopenic purpura).

Why does FNAIT happen?

Platelets have many proteins on their surface. Each person has a different set of proteins, which are inherited from their parents. Sometimes a baby inherits a protein that is found on their father’s platelets but is absent from their mother’s platelets. When this happens the mother's immune system may respond by developing an antibody to fight against the protein the baby inherited from their father. This is called an alloimmune response.

The antibody may pass from the mother’s blood into the baby’s blood through the placenta and attach to the baby’s platelets. This antibody destroys the baby’s platelets, resulting in a low platelet count and an increased risk of bleeding for the baby.

The mother’s antibodies can remain in the baby’s blood stream for weeks, and bleeding can occur in the baby before birth (fetal), during birth or after birth (neonatal).

There are a number of proteins that can cause FNAIT but the most common one is human platelet antigen (HPA)-1a.

FNAIT does not occur because of anything the mother does, eats or drinks. It has not been linked to any environmental or dietary exposures.

How common is FNAIT?

FNAIT is rare. It occurs in about one in every 2,000 births.

How is FNAIT diagnosed?

Doctors may suspect a baby has FNAIT if there is:

• bleeding or bruising
• a low platelet count on a blood test after birth
• a specific pinpoint rash (called petechiae)

The diagnosis of FNAIT can be confirmed by taking blood samples from the mother, the father and possibly the baby. If the baby’s platelet count is very low, doctors may look for possible bleeding in the brain using ultrasound.

Why blood tests in pregnancy do not show that the baby may have FNAIT?

Because FNAIT is rare, doctors do not usually screen for it. Doctors may first suspect a baby has FNAIT when they are born with bruises or a rash.

How is FNAIT treated?

Babies with FNAIT may need to be admitted to the neonatal intensive care unit for close monitoring.

The main treatment for FNAIT is platelet transfusions. These transfusions will help to prevent and treat the bleeding.

What can be done to prevent FNAIT in future pregnancies?

It is possible that FNAIT will occur in future pregnancies. This will depend on which platelet proteins the baby inherits from their mother and father.

If a woman becomes pregnant again, their doctor should refer them to a hospital that specializes in fetal-maternal medicine. If a woman is known to be at risk of FNAIT, their pregnancy should be followed by a multidisciplinary team of specialists including an obstetrician and a hematologist with expertise in FNAIT.

Doctors will closely monitor the pregnancy and may start treatment as early as 12 to 16 weeks of gestation. This treatment includes intravenous immunoglobulin (IVIG) and possibly steroids. Treatment is based on several factors that should be discussed with their doctor. Treatment will likely be needed to prevent a low platelet count in the fetus and newborn.

Does FNAIT affect the mother during pregnancy?

FNAIT does not directly affect the mother’s health, but once FNAIT is diagnosed in a baby, the mother’s future pregnancies should be followed by a specialist who may recommend preventative treatments for the mother.

Should the mother's brother or sister be tested for FNAIT?

A woman's sisters may have the same rare platelet protein combination and therefore should be tested. Other relatives do not need to be tested.

What to expect

If a baby has FNAIT can they still be breastfed?

If the baby's pediatrician says they are well enough to feed then breastfeeding them is safe.

Will a baby with FNAIT always be at risk of having a low platelet count as they grow older?

No, a baby's low platelet count should only last until a few weeks after their birth.

Does having FNAIT mean a baby will have a weak immune system when they are older?

No. There is no known association between FNAIT and a weak immune system.

Where can I get more information?

Visit www.naitbabies.org.

This document was created by the International Collaboration for Transfusion Medicine Guidelines (ICTMG), a group of international transfusion experts brought together to create and promote evidence-based guidelines to optimize transfusion care. For more information, visit the website at www.ictmg.org.